Electrical converters, in particular in the medium and high voltage area, are used for converting a first current with a first frequency and a first voltage into a second current with a second frequency and a second voltage. Many types of converters are known such as for converting AC to AC, AC to DC, DC to AC and DC to DC.
Usually, converters comprise high power semiconductors for switching internally currents to produce the desired output current. In modular converters, these power semiconductors are distributed among converter modules, which also may comprise further components like a controller for the semiconductors or a capacitor for storing energy in the converter module.
For example, the M2LC topology has become popular in both medium and high voltage applications. An M2LC converter or modular multi-level converter comprises converter arms with converter modules connected in series for generating a multi-level output voltage. In an M2LC converter the converter modules each comprise a capacitor themselves.
The standard approach to achieve closed-loop control for an M2LC converter is to divide the control problem into two hierarchical layers. The upper layer is based on vector control with a modulator. The vector control scheme operates in a orthogonal reference frame rotating with a certain angular velocity. By manipulating the voltage reference to the modulator, closed-loop control of the load currents can be achieved. Typically, carrier-based pulse width modulation (PWM) or space vector modulation (SVM) is used as modulator. The circulating currents and/or the energy balance within the converter arms may be addressed by adding additional control loops.
The lower control layer utilizes the redundancy in the converter states (e.g. groups of switching states that produce the same line to line voltage, and/or groups of switching states that produce the same arm voltage) in order to balance the capacitor voltages. The capacitor voltages are sorted in an ascending/descending order of their voltage values. For a charging current the capacitors with the lowest voltages are selected first, and conversely, the capacitors with the highest voltages are prioritized for discharging currents.